Ossified tendons

Let’s face it, although it would be far less catchy it would be more accurate to call this the ‘theropod and pterosaur and occasionally other things’ blog more than the Archosaur Musings, but since that is where my knowledge and interests lie, that’s what you tend to get. However I am trying to at least get some more ornithischians, birds and crocs in here but bear with me if it takes a while and tends to be short.

Here though I want to mention one defining characteristic of the Ornithischia as it actually relates to both theropods and pterosaurs, if rather obliquely, namely the ossified tendons of the spine. Many of you are probably familiar with these already since they crop up in various ecological and mechanical discussions about how tails work (especially with the old one about hadrosaurs swimming). For those who don’t however, the short version is that ornithischians have a series of elongate little bones that run alongside the vertebral column, especially close to the sacrum.

I say ‘bones’ and in a sense they are, though they are not like normal skeletal elements but are tendons that have been turned into bones. This is actually more familiar that you might think – you may well have noticed how tough some tendons are in a turkey carcass compared to a chicken and this is a result of the same process. Ossified tendons are quite literally those that are partially, or fully turned to bony tissue. Those tendons that take heavy loads often have this happen to them as seen in turkeys or for an extinct examplw the nyctosaurid pterosaurs.

These are present in pretty much all ornithischians (stegosaurs don’t have them), though don’t always show up in juvenile animals and some have far more than others. They are typically concentrated on the vertebrae of the sacrum but do appear on the dorsals and caudals too in most cases. They are typically long and thin and lie subparallel to each other, though famously in the hadrosaurs they form a lattice work on the tail. Here’s a not very good image showing then in Psittacosaurus (above) and Yinlong (below). In the former they appear as a series of stripes and in the latter as some rods (though badly broken) but between the two it should give you an idea of what I mena and demonstrate that they are not part of the vertebrae themselves.

So, where’s the relevance to dromaeosaurs and pterosaurs? Well in some cases, (with Tianyulong being a notable example) there can be loads of ossified tendons binding the tail up in a tight mass of little splints of bone. This can look very similar to the situation in both dromaeosaurs and some rhamphorhynchoid pterosaurs where the tail is similarly bound up. However, in the latter cases this is a result of those (normally) little articulations on the vertebrae (the pre and postzygopophyses) being enormously extended into long thin rods. The tails can therefore look superficially similar between the two groups, but close examination should reveal the fact that in the ornithischians, the rods of bone are not part of the vertebrae by lying on top of them. It’s actually a rather nice demonstration of solving the same problem (stiffening the tail in this case) in two different ways as well as the convergence between the dromaeosaur and rhamphorhynchoid tails.